Space time block coded transmit antenna diversity for WCDMA

ABSTRACT

A mobile communication system is designed with an input circuit coupled to receive a first plurality of signals (r j (i+τ j ), i=0−N−1) during a first time (T 0 -T 1 ) from an external source and coupled to receive a second plurality of signals (r j (i+τ j ), i=N−2N−1) during a second time (T 1 -T 2 ) from the external source. The input circuit receives each of the first and second plurality of signals along respective first and second paths (j). The input circuit produces a first input signal (R j   1 ) and a second input signal (R j   2 ) from the respective first and second plurality of signals. A correction circuit is coupled to receive a first estimate signal (α j   1 ), a second estimate signal (α j   2 ) and the first and second input signals. The correction circuit produces a first symbol estimate ({tilde over (S)} 1 ) in response to the first and second estimate signals and the first and second input signals. The correction circuit produces a second symbol estimate ({tilde over (S)} 2 ) in response to the first and second estimate signals and the first and second input signals.

CLAIM TO PRIORITY

This application is a division of nonprovisional application Ser. No.09/205,029, filed Dec. 3, 1998 and provisional application No.60/103,443, filed Oct. 7, 1998.

FIELD OF THE INVENTION

This invention relates to wideband code division multiple access (WCDMA)for a communication system and more particularly to space time blockcoded transmit antenna diversity for WCDMA.

BACKGROUND OF THE INVENTION

Present code division multiple access (CDMA) systems are characterizedby simultaneous transmission of different data signals over a commonchannel by assigning each signal a unique code. This unique code ismatched with a code of a selected receiver to determine the properrecipient of a data signal. These different data signals arrive at thereceiver via multiple paths due to ground clutter and unpredictablesignal reflection. Additive effects of these multiple data signals atthe receiver may result in significant fading or variation in receivedsignal strength. In general, this fading due to multiple data paths maybe diminished by spreading the transmitted energy over a wide bandwidth.This wide bandwidth results in greatly reduced fading compared to narrowband transmission modes such as frequency division multiple access(FDMA) or time division multiple access (TDMA).

A CDMA spread spectrum (“SS”) signal is created by modulating the radiofrequency (“RF”) signal with a spreading sequence (a code consisting ofa series of binary pulses) known as a pseudo-noise (“PN”) digital signalbecause they make the signal appear wide band and “noise like”. The PNcode runs at a higher rate than the RF signal and determines the actualtransmission bandwidth. The resulting signal has a low-power spectraldensity in any narrow portion of the band. Messages can becryptographically encoded to any level of secrecy desired with directsequencing as the entire transmitted/received message is purely digital.

New standards are continually emerging for next generation wideband codedivision multiple access (WCDMA) communication systems as described inProvisional U.S. Patent Application No. 60/082,671, filed Apr. 22, 1998,and incorporated herein by reference. These WCDMA systems are coherentcommunications systems with pilot symbol assisted channel estimationschemes. These pilot symbols are transmitted as quadrature phase shiftkeyed (QPSK) known data in predetermined time frames to any receiverswithin range. The frames may propagate in a discontinuous transmission(DTX) mode. For voice traffic, transmission of user data occurs when theuser speaks, but no data symbol transmission occurs when the user issilent. Similarly for packet data, the user data may be transmitted onlywhen packets are ready to be sent. The frames include pilot symbols aswell as other control symbols such as transmit power control (TPC)symbols and rate information (RI) symbols. These control symbols includemultiple bits otherwise known as chips to distinguish them from databits. The chip transmission time (T_(c)), therefore, is equal to thesymbol time rate (T) divided by the number of chips in the symbol (N).

Previous studies have shown that multiple transmit antennas may improvereception by increasing transmit diversity for narrow band communicationsystems. In their paper New Detection Schemes for Transmit Diversitywith no Channel Estimation, Tarokh et al. describe such a transmitdiversity scheme for a TDMA system. The same concept is described in ASimple Transmitter Diversity Technique for Wireless Communications byAlamouti. Tarokh et al. and Alamouti, however, fail to teach such atransmit diversity scheme for a WCDMA communication system.

Other studies have investigated open loop transmit diversity schemessuch as orthogonal transmit diversity (OTD) and time switched timediversity (TSTD) for WCDMA systems. Both OTD and TSTD systems havesimilar performance. Both use multiple transmit antennas to provide somediversity against fading, particularly at low Doppler rates and whenthere are insufficient paths for the rake receiver. Both OTD and TSTDsystems, however, fail to exploit the extra path diversity that ispossible for open loop systems. For example, the OTD encoder circuit ofFIG. 5 receives symbols S₁ and S₂ on lead 500 and produces outputsignals on leads 504 and 506 for transmission by first and secondantennas, respectively. These transmitted signals are received by adespreader input circuit (FIG. 6). The input circuit receives the i^(th)of N chip signals per symbol together with noise along the j^(th) of Lmultiple signal paths at a time τ_(j) after transmission. Both here andin the following text, noise terms are omitted for simplicity. Thisreceived signal r_(j)(i+τ_(j)) at lead 600 is multiplied by a channelorthogonal code signal C_(m)(i+τ_(j)) that is unique to the receiver atlead 604. Each chip signal is summed over a respective symbol time bycircuit 608 and produced as first and second output signals R_(j) ¹ andR_(j) ² on leads 612 and 614 as in equations [1-2], respectively. Delaycircuit 610 provides a one-symbol delay T so that the output signals areproduced simultaneously. $\begin{matrix}{R_{j}^{1} = {{\sum\limits_{i = 0}^{N - 1}{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{1}} + {\alpha_{j}^{2}S_{2}}}}} & \lbrack 1\rbrack \\{R_{j}^{2} = {{\sum\limits_{i = N}^{{2N} - 1}{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{1}} - {\alpha_{j}^{2}S_{2}}}}} & \lbrack 2\rbrack\end{matrix}$

The OTD phase correction circuit of FIG. 7 receives the signals R_(j) ¹and R_(j) ² as input signals corresponding to the j^(th) of L multiplesignal paths. The phase correction circuit produces soft outputs orsignal estimates {tilde over (S)}₁ and {tilde over (S)}₂ for symbols S₁and S₂ at leads 716 and 718 as shown in equations [3-4], respectively.$\begin{matrix}{{\overset{\sim}{S}}_{1} = {{\sum\limits_{j = 1}^{L}{\left( {R_{j}^{1} + R_{j}^{2}} \right)\quad \alpha_{j}^{1*}}} = {\sum\limits_{j = 1}^{L}{2{\alpha_{j}^{1}}^{2}S_{1}}}}} & \lbrack 3\rbrack \\{{\overset{\sim}{S}}_{2} = {{\sum\limits_{j = 1}^{L}{\left( {R_{j}^{1} - R_{j}^{2}} \right)\quad \alpha_{j}^{2*}}} = {\sum\limits_{j = 1}^{L}{2{\alpha_{j}^{2}}^{2}S_{2}}}}} & \lbrack 4\rbrack\end{matrix}$

Equations [3-4] show that the OTD method provides a single channelestimate α for each path j. A similar analysis for the TSTD systemyields the same result. The OTD and TSTD methods, therefore, are limitedto a path diversity of L. This path diversity limitation fails toexploit the extra path diversity that is possible for open loop systemsas will be explained in detail.

SUMMARY OF THE INVENTION

These problems are resolved by a mobile communication system comprisingan input circuit coupled to receive a first plurality of signals duringa first time from an external source and coupled to receive a secondplurality of signals during a second time from the external source. Theinput circuit receives each of the first and second plurality of signalsalong respective first and second paths. The input circuit produces afirst input signal and a second input signal from the respective firstand second plurality of signals. A correction circuit is coupled toreceive a first estimate signal, a second estimate signal and the firstand second input signals. The correction circuit produces a first symbolestimate in response to the first and second estimate signals and thefirst and second input signals. The correction circuit produces a secondsymbol estimate in response to the first and second estimate signals andthe first and second input signals.

The present invention improves reception by providing at least 2Ldiversity over time and space. No additional transmit power or bandwidthis required. Power is balanced across multiple antennas.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be gained by readingthe subsequent detailed description with reference to the drawingswherein:

FIG. 1 is a simplified block diagram of a typical transmitter usingSpace Time Transit Diversity (STTD) of the present invention;

FIG. 2 is a block diagram showing signal flow in an STTD encoder of thepresent invention that may be used with the transmitter of FIG. 1;

FIG. 3 is a schematic diagram of a phase correction circuit of thepresent invention that may be used with a receiver;

FIG. 4A is a simulation showing STTD performance compared to TimeSwitched Time Diversity (TSTD) for a vehicular rate of 3 kmph;

FIG. 4B is a simulation showing STTD performance compared to TSTD for avehicular rate of 120 kmph;

FIG. 5 is a block diagram showing signal flow in an OTD encoder of theprior art;

FIG. 6 is a block diagram of a despreader input circuit of the priorart; and

FIG. 7 is a schematic diagram of a phase correction circuit of the priorart.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is a simplified block diagram of a typicaltransmitter using Space Time Transit Diversity (STTD) of the presentinvention. The transmitter circuit receives pilot symbols, TPC symbols,RI symbols and data symbols on leads 100, 102, 104 and 106,respectively. Each of the symbols is encoded by a respective STTDencoder as will be explained in detail. Each STTD encoder produces twooutput signals that are applied to multiplex circuit 120. The multiplexcircuit 120 produces each encoded symbol in a respective symbol time ofa frame. Thus, a serial sequence of symbols in each frame issimultaneously applied to each respective multiplier circuit 124 and126. A channel orthogonal code C_(m) is multiplied by each symbol toprovide a unique signal for a designated receiver. The STTD encodedframes are then applied to antennas 128 and 130 for transmission.

Turning now to FIG. 2, there is a block diagram showing signal flow inan STTD encoder of the present invention that may be used with thetransmitter of FIG. 1. The STTD encoder receives symbol S₁ at symboltime T and symbol S₂ at symbol time 2T on lead 200. The STTD encoderproduces symbol S₁ on lead 204 and symbol −S*₂ on lead 206 at symboltime T, where the asterisk indicates a complex conjugate operation.Furthermore, the symbol time indicates a relative position within atransmit frame and not an absolute time. The STTD encoder then producessymbol S₁ on lead 204 and symbol S*₁ on lead 206 at symbol time 2T. Thebit or chip signals of these symbols are transmitted serially alongrespective paths 208 and 210. Rayleigh fading parameters are determinedfrom channel estimates of pilot symbols transmitted from respectiveantennas at leads 204 and 208. For simplicity of analysis, a Rayleighfading parameter α_(j) ¹ is assumed for a signal transmitted from thefirst antenna 204 along the j^(th) path. Likewise, a Rayleigh fadingparameter α_(j) ² is assumed for a signal transmitted from the secondantenna 206 along the j^(th) path. Each i^(th) chip or bit signalr_(j)(i+τ_(j)) of a respective symbol is subsequently received at aremote mobile antenna 212 after a transmit time τ_(j) corresponding tothe j^(th) path. The signals propagate to a despreader input circuit(FIG. 6) where they are summed over each respective symbol time toproduce output signals R_(j) ¹ and R_(j) ² corresponding to the j^(th)of L multiple signal paths as previously described.

Referring now to FIG. 3, there is a schematic diagram of a phasecorrection circuit of the present invention that may be used with aremote mobile receiver. This phase correction circuit receives signalsR_(j) ¹ and R_(j) ² as input signals on leads 610 and 614 as shown inequations [5-6], respectively. $\begin{matrix}{R_{j}^{1} = {{\sum\limits_{i = 0}^{N - 1}{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{1}} - {\alpha_{j}^{2}S_{2}^{*}}}}} & \lbrack 5\rbrack \\{R_{j}^{2} = {{\sum\limits_{i = N}^{{2N} - 1}{r_{j}\left( {i + \tau_{j}} \right)}} = {{\alpha_{j}^{1}S_{2}} + {\alpha_{j}^{2}S_{1}^{*}}}}} & \lbrack 6\rbrack\end{matrix}$

The phase correction circuit receives a complex conjugate of a channelestimate of a Rayleigh fading parameter α_(j) ^(1*) corresponding to thefirst antenna on lead 302 and a channel estimate of another Rayleighfading parameter α_(j) ² corresponding to the second antenna on lead306. Complex conjugates of the input signals are produced by circuits308 and 330 at leads 310 and 322, respectively. These input signals andtheir complex conjugates are multiplied by Rayleigh fading parameterestimate signals and summed as indicated to produce path-specific firstand second symbol estimates at respective output leads 318 and 322 as inequations [7-8].

R _(j) ¹α_(j) ^(1*) +R _(j) ^(2*)α_(j) ²=(|α_(j) ¹|²+|α_(j) ²|²)S ₁  [7]

−R _(j) ^(1*)α_(j) ² +R _(j) ²α_(j) ^(1*)=(|α_(j) ¹|²+|α_(j) ²|²)S₂  [8]

These path-specific symbol estimates are then applied to a rake combinercircuit to sum individual path-specific symbol estimates, therebyproviding net soft symbols as in equations [9-10]. $\begin{matrix}{{\overset{\sim}{S}}_{1} = {{\sum\limits_{j = 1}^{L}{R_{j}^{1}\quad \alpha_{j}^{1*}}} + {R_{j}^{2*}\alpha_{j}^{2}}}} & \lbrack 9\rbrack \\{{\overset{\sim}{S}}_{2} = {{\sum\limits_{j = 1}^{L}{{- R_{j}^{1*}}\quad \alpha_{j}^{2}}} + {R_{j}^{2}\alpha_{j}^{1*}}}} & \lbrack 10\rbrack\end{matrix}$

These soft symbols or estimates provide a path diversity L and atransmit diversity 2. Thus, the total diversity of the STTD system is2L. This increased diversity is highly advantageous in providing areduced bit error rate. The simulation result of FIG. 4 compares a biterror rate (BER) of STTD with TSTD for various ratios of energy per bit(Eb) to noise (No) at a relative speed of 3 Kmph. The OTD and TSTDsystems were found to be the same in other simulations. The simulationshows that a 7.5 dB ratio Eb/No corresponds to a BER of 2.0E-3 for TSTD.The same BER, however, is achieved with a 7.2 dB ratio Eb/No. Thus, STTDproduces approximately 0.3 dB improvement over TSTD. The simulation ofFIG. 5 compares the BER of STTD with TSTD for various values of Eb/No ata relative speed of 120 Kmph. This simulation shows a typical 0.25 dBimprovement for STTD over TSTD even for high Doppler rates. By way ofcomparison, STTD demonstrates a 1.0 dB advantage over the simulatedcurve of FIG. 5 without diversity at a BER of 2.6E-3. This substantialadvantage further demonstrates the effectiveness of the presentinvention.

Although the invention has been described in detail with reference toits preferred embodiment, it is to be understood that this descriptionis by way of example only and is not to be construed in a limitingsense. For example, several variations in the order of symboltransmission would provide the same 2L diversity. Moreover, theexemplary diversity of the present invention may be increased with agreater number of transmit or receive antennas. Furthermore, novelconcepts of the present invention are not limited to exemplarycircuitry, but may also be realized by digital signal processing as willbe appreciated by those of ordinary skill in the art with access to theinstant specification.

It is to be further understood that numerous changes in the details ofthe embodiments of the invention will be apparent to persons of ordinaryskill in the art having reference to this description. It iscontemplated that such changes and additional embodiments are within thespirit and true scope of the invention as claimed below.

What is claimed:
 1. A circuit, comprising: a plurality of encodercircuits, each encoder circuit coupled to receive a respective inputsignal, each encode circuit arranged to produce a respective encodedoutput signal at least one encoded output signal including a firstsymbol, a second symbol, a first transformed symbol corresponding to thefirst symbol, and a second transformed symbol corresponding to thesecond symbol; and a plurality of transmit antennas coupled to receivethe fit and the second symbols and the first and second transformedsymbols.
 2. A circuit as in claim 1, further comprising a multiplexcircuit coupled to receive the respective encoded output signal fromeach of the plurality of encoder circuits, the multiplex circuitselectively producing the first and a second symbols and the first andsecond transformed symbols.
 3. A circuit as in claim 2, wherein thefirst and second symbols are applied to a first of the plurality oftransmit antennas and wherein the first and second transformed symbolsare applied to a second of the plurality of transmit antennas.
 4. Acircuit as in claim 3, wherein each transformed symbol includes at leastone of a complement and a complex conjugate of a respective symbol.
 5. Acircuit as in claim 3, wherein at least one of the symbols applied tothe plurality of transmit antennas is modulated by a channel orthogonalcode.
 6. A circuit as in claim 1, further comprising a multipliercircuit coupled to receive a code and at least one of said symbols, themultiplier circuit arranged to multiply the code by the at least one ofsaid symbols.
 7. A circuit as in claim 6, wherein the code is a channelorthogonal code.
 8. A circuit as in claim 7, wherein the channelorthogonal code comprises a Walsh code.
 9. A circuit as in claim 9,wherein the code comprises a scrambling code.
 10. A circuit as in claim6, wherein the code corresponds to only one remote receiver.
 11. Acircuit as in claim 1, wherein said respective input signal comprises atleast one of a pilot symbol, transmit power control symbol, rateinformation symbol and data symbol.
 12. A circuit as in claim 9, whereinthe scrambling code comprises a pseudo noise code.
 13. A circuit as inclaim 1, wherein the first and the second symbols are quadrature phaseshift keyed symbols.
 14. A method of processing signals comprising thesteps of: encoding an input data sequence thereby producing an encodeddata sequence; interleaving the encoded data sequence, thereby producingan interleaved data sequence; producing a plurality of symbols, from theinterleaved data sequence; applying a first symbol to a first antenna ata first time; apply a second symbol to a second antenna at the firsttime; applying a third symbol to the first antenna at a second time; andapplying a fourth symbol to the second antenna at the second time,wherein each of the first through fourth symbols is different from theothers of the first trough fourth symbols, wherein the second symbol isa conjugate of the third symbol and wherein the fourth symbol is anegative of a conjugate of the first symbol.
 15. A method as in claim14, wherein the input data sequence comprises a sequence of data bits.16. A method as in claim 15, wherein the encoded data sequence isgreater in number than the sequence of input data bits.
 17. A method asin claim 14, wherein the encoded data sequence is encoded with aconvolutional code.
 18. A method as in claim 14, wherein the encodeddata sequence is encoded with a turbo code.
 19. A method as in claim 14,wherein the interleaved data sequence comprises data bits of the encodeddata sequence having a different order than the encoded data sequence.20. A method as in claim 14, wherein each of the plurality of symbols isa quadrature phase shift keyed symbol.
 21. A method as in claim 20,wherein said quadrature phase shift keyed symbol comprises an inphasedata bit and a quadrature data bit from the interleaved data sequence.22. A method as in claim 14, comprising the step of multiplying each ofthe plurality of symbols by a channel orthogonalization code.
 23. Amethod of processing signals, comprising the steps of: encoding an inputdata sequence thereby producing an encoded data sequence; interleavingthe encoded data sequence, thereby producing an interleaved datasequence; producing a plurality of symbols, from the interleaved datasequence; applying a first symbol to a first antenna at a first time;applying a second symbol to a second antenna at the first time; applyinga third symbol to the first antenna at a second time; and applying afourth symbol to the second antenna at the second time, wherein each ofthe first through fourth symbols is different from the others of thefirst through fourth symbols, wherein the second symbol is a conjugateof the third symbol and wherein the first symbol is a negative of aconjugate of the fourth symbol.